Industrial wastewater contains a host of pollutants, organic and inorganic types. The complexity of wastewater composition often demands application of advanced processes for achieving the desirable effluent quality and for improving the treatment efficiency of one or more intermediate process stages. Among the many approaches, electrochemical methods have been regarded as suitable for the treatment objectives of different types of wastewaters. It has been shown that many organic impurities in aqueous phase can be removed effectively by direct or indirect electrochemical oxidation. The electro oxidation applications primarily involve very specific anodic reactions to generate specific oxidants such as hydroxyl radicals at the polarized electrodes (anode) which is referred as ‘direct’ oxidation. Hydroxyl free radical is a very powerful, non-specific oxidizing species which attacks most organic molecules as well as oxidizable inorganic molecules and ions. Hydroxyl radicals may be produced by irradiation with ultraviolet light of particles of titanium dioxide dispersed in water or by reaction of hydrogen peroxide with iron salts dissolved in mildly acidic solution, called Fenton's reaction. Redox species such as Cl2/OCl− can also cause a type of oxidation called ‘indirect’ oxidation.
It is found that electro-oxidation of pollutants in bi-polar cell comprising of different anode materials was by far the most studied systems. In this type of cells, anode and cathode plates are arranged parallel to each other at specific distance, called inter electrode spacing, allowing them to come in contact with liquid effluent and applying potential for a specific period to afford contaminant removal. In this type of parallel plate design, the stability of anode materials, poor mass transfer as well as reaction rates, and longer reaction times are of paramount importance. The lack of stable and inexpensive anodes and the need for mixing devices and accompanying higher costs are more of concern in the parallel plate configuration. Among the many approaches to overcome the mass transfer limitations in such reactors is the use of electrodes that comprise particulate materials are known. Since these electrodes possess higher surface area, the reactors including them are expected to have high surface area to volume ratio, and hence can give rise to higher rates of reaction. These electrodes are referred as ‘3-D electrodes’ in literature. More recently, cells employing three-dimensional electrodes, such as bulk/block electrodes, were used for enhancing the treatment efficiency. The three dimensional electrode reactors can yield higher treatment efficiency due to higher surface area-to-volume ratio. However, the prior art using 3-D electrodes was developed to address mainly the cathodic processes for recovery of metals by deposition of a metal from an aqueous solution. Because these 3-D electrodes comprised mainly particulate materials e.g., graphite, carbon, and metals in different shapes and forms considering the process constraints dictated by cathode environment. The same 3-D electrodes can not be used for driving anodic processes with high energy efficiency and contaminant removal efficiency, particularly under high applied current. Fundamental studies on the kinetics and mechanism of the purification of wastewaters in different types of electrochemical reactors can be found in the literature. The research mainly relates to development of electrodes, devices and methods for electrochemically producing oxidants, disinfectants, metal hydroxides, etc. for removing or decomposing the specific chemical compounds and microbial pollutants.
Prior-art search was made in patent as well as non-patent literature. Following references are referred due to their relevance to the field of present invention.
U.S. Pat. No. 3,793,173 (1974) describes “wastewater treatment using electrolysis with activated carbon cathode”, for production of hydrogen peroxide through oxygen reduction at hollow porous carbon cathode. The hydrogen peroxide reacts with organic pollutants in wastewater which thereby gets treated. The system requires injection of oxygen in the cathode region or requires maintaining high concentration of dissolved oxygen in wastewater for obtaining higher rates of removal of organic contaminants. In this art, only the cathode region is of three-dimensional nature.
U.S. Pat. No. 3,919,062 (1975) describes “electrochemical system graduated porous bed sections”, that is claimed to be useful for pollution control, electrochemical synthesis and metal recovery. According to this invention, the reaction zone contains electrically conductive particles, such as carbon pellets disposed between primary electrodes and electrolyte flows in a direction parallel to voltage gradient between the primary electrodes. The reactor has graduated sections of porous bed having different thickness; the sections were separated by insulating spacers. However, the reactor is claimed to be capable of handling large quantities of dilute electrolyte solutions, especially effective for removal of metal ions from dilute aqueous solutions.
U.S. Pat. No. 3,915,822 (1975) describes “electrochemical system with bed sections is having variable gradient” according to which electrochemical reaction zones were built using three primary electrodes and electrically conductive discrete particles contained in the reaction zones. The system establishes different voltage gradients across adjacent sections of the reaction zones. Electrolyte can be circulated through the sections serially and homogeneous treatment of wastes for metal recovery can be obtained. This type of bed reactor has a major disadvantage that it requires more than one DC source to create sections having variable voltage gradient. Further, the use of the reactor for treating wastewater containing high concentrations of organic impurities has not been tested.
U.S. Pat. No. 4,585,539 (1986) discloses “Electrolytic reactor”, according to which a sealed volume reactor wherein two separate chambers are associated with the cathode and anode, the two chambers separated by micro-porous electrically insulating septum, one of the chambers filled with electrically conductive particulate material-stainless steel balls-acting as extensions of the electrode in that chamber. The device can be used for recovery of metallic impurities from dilute solutions. However, due to use of insulating septum (ceramic) to separate the chambers the system would require application of higher voltages and use of stainless steel balls in anode region may be leading to their dissolution. Further, the invention did not look at the potential of the device to degrade organic contaminants.
U.S. Pat. No. 5,549,812 (1996) described “process and apparatus for processing industrial wastewater by electrolysis” according to which a cascade type arrangement of electrodes was provided for treatment of only contaminants susceptible for precipitation and sedimentable flocculates. The principle of operation mainly depends on the ‘electrofloatation’. This art employs Al or Fe electrode plates defined by ribs or strips on one plate against which a flat surface of the other plate can be fixed. These electrodes undergo dissolution releasing metal ions as flocculating agents which adsorb organic compounds and subsequently/settle as the case may be. The main purpose of such an electrode arrangement with narrow inter electrode spacing is not only to allow liquid to flow in a meandering path, but also to increase flow velocity by constricting the flow with the help of ribs and insulating guide strips. Moreover the art demands application of pulse DC current and also pulse flow of water.
U.S. Pat. No. 5,690,806 (1997) discussed “cell and method for the recovery of metals from dilute solutions” according to which the cell comprises a porous carbon fiber material placed in porous tubular support as cathode. This along with a tubular anode encased in non-porous outer casing functions to recover valuable metals from wastes. Again, this invention relates to the recovery of metals from dilute solutions and does not address removal of organic pollutants. The possibility of using carbon fiber material stuffed in porous tubular support as 3-D anode was not envisaged in this art.
U.S. Pat. No. 5,702,587 (1997) discloses a method for “chemical and electrochemical regeneration of active carbon” wherein an apparatus for regenerating active carbon subsequent to saturation by use and a method for enhancing the adsorption capacity of active carbon is described. In accordance with this invention the reaction of ferrous ions with electro generated hydrogen peroxide in an electrolytic cell having carbon cathode containing adsorbed, oxidizable, organic materials and a conventional metallic anode that allows oxidation of oxidizables. The method employs transition metal salts or chelate of a transition metal ion to aid hydroxyl radical formation from electro generated hydrogen peroxide and requires external supply of oxygen and expensive anodes. The system suffers from certain issues like rate limiting by oxygen diffusion, tested with distilled water having low concentration of phenol, requires intermittent pH adjustment, requires longer operating times as high as 60 h, and involves multiple cycles of separate operations, viz. adsorption, desorption and destruction.
U.S. Pat. No. 5,744,028 (1998) describes “water treating apparatus” which incorporates an electrolytic cell including an anode, cathode and a three-dimensional carbon electrode provided between the anode and the cathode in a water dispensing apparatus. This system was tested useful for the removal of micro organisms and thereby water gets sterilized. However, the system suffers from the need for keeping the power supply on continuously for obtaining sterilization effect. The system can not be scaled up easily for effluent treatment systems having to treat large concentrations of organic compounds.
U.S. Pat. No. 5,770,037 (1998) describes “water processing method”, using an electrolytic cell including an anode, a cathode, and a three-dimensional carbon electrode provided between the anode and cathode, for sterilizing water. It is stated that water to be treated should have at least 10 MΩ·cm resistivity and inter-electrode separation is not more than, 1.0 mm. However, the 3-D fixed bed type of reactor has not been tested for removal of pollutants from effluents. Moreover, due to very less inter-electrode separation the size of 3-D carbon bed is also limited, thereby limiting throughput volume of treated water.
U.S. Pat. No. 6,298,996 (2001) discusses a “three dimensional electrode for electrolytic removal of contaminants from aqueous waste streams”. According to this invention, high surface area charged metal particles are constantly in motion in the spouted cathode area, into this area a jet of stream containing the trace heavy metal contaminants is introduced whereby deposition of heavy metals on the charged metal particles takes place. While the spouted electrode design provided for lowering mass transfer limitations and increasing geometric current densities, the use of metal particles as charged anodes in anode area, similar to those in cathode area is seriously limited due to the possibility of corrosion and anodic dissolution and therefore anodic reactions can not be effectively carried out using such spouted electrode systems and hence oxidative removal of organic contaminants can not be done using the apparatus described in this patent.
U.S. Pat. No. 6,274,028 (2001) discloses “electrolytic wastewater treatment method and apparatus” describes a method and apparatus for purifying aqueous effluent streams to reduce contamination as measured by chemical oxygen demand, where the method comprises direct oxidation of water-soluble organic and oxidizable inorganic substances in an electrolytic oxidation cell that incorporates stainless steel electrodes, and wherein the stability and lifetime of the anode are enhanced by incorporation of metal chips. While, the system is claimed to be useful for wastewaters having chemical oxygen demand in the range 200-2000 ppm, the use of stainless steel anodes and iron chips leads to sludge and flocs generation which needs to be separated. Since a portion of organic compounds get removed adsorbed on the surface of sludge and flocs, this will have to be treated as any other hazardous sludge requiring special disposal methods.
A patent WO 2004/079840 A2 (2004) discusses “three-dimensional flow-through electrode and electrochemical cell” useful for water treatment, and environmental cleanup. The three-dimensional flow-through electrode includes an efficient current feeding mechanism including current feeders comprising rods of conductive material, such as graphite, which are inserted into a block of graphite felt. The invention specifically addresses the method of feeding electrical current into a three-dimensional electrode composed of graphite felt and does not aim for achieving higher surface area to volume ratios required for efficient treatment of effluents.
In order to overcome problems associated with prior art, there is need for intensification of electrolytic treatment process of liquid effluents which will avoid or minimize the above described problems. The intensification will desirably have one or many features as follows: i) relatively inexpensive anode materials, viz. high surface area granular activated carbon, instead of noble metal oxide stabilized titanium anodes or otherwise ii) relatively simple design, viz. a tank type reactor instead of sealed, concentric cylindrical and spouted electrode designs iii) a single channel flow of liquid effluent having sufficiently longer flow path, iv) both anode and cathode are 3-D type, instead of only one v) without any insulating plastic or ceramic barriers between anode and cathode, and only spatial separation and contact and electrolytic resistance in the reaction zones should be adequate vi) high rate of contaminant removal from high organic strength effluents, vii) lower power consumption, viii) smaller reactor footprint compared to the conventional unit processes in the effluent treatment, e.g., chemical precipitation, clariflocculation, biooxidation etc. No prior art presented above is able to oxidize high concentrations of organic substrates, or oxidize in a non-specific manner a vide variety of chemical substances present in actual wastewaters leading to overall improvement in the effluent quality. Moreover, the potential of an electrochemical method, particularly electro oxidation for treating actual wastewater such as chemical industry wastewater comprising large concentrations of several non-specific pollutants and having very high concentrations of Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD), Total Kjeldahl Nitrogen (TKN) and Total Dissolved Solids (TDS), has not been explored.